The subject invention generally relates to a brake system and method having secondary hydraulic braking in a de-energized operational mode. More specifically, the subject invention relates to a brake system and method for vehicles using solenoid-controlled hydraulic braking.
xe2x80x9cBrake-by-wirexe2x80x9d (BBW) brake systems are known in the art. Furthermore, a developing technological subset of BBW brake systems is known in the art as xe2x80x9cdry interfacexe2x80x9d brake systems. In a typical dry interface brake system, driver input is transmitted to the brake system electronically through a controller such as an electronic control unit, rather than through conventional mechanical and hydraulic transmission of driver input. The driver input is transmitted by the controller to brake application devices at each corner. As understood by those skilled in the art, corner is a term used throughout the industry to describe all of the equipment that is used at the vehicle wheel assembly, including, but not limited to, the brake, the suspension, and the drive train systems.
Although dry interface brake systems eliminate some of the hydraulic components associated with standard hydraulic brake systems, it should be noted that these systems are not completely non-hydraulic or xe2x80x9cdry.xe2x80x9d Unlike standard hydraulic brake systems, which utilize vacuum-assisted driver brake pedal force to generate the pressure necessary to apply the brakes, dry interface brake systems include actuator assemblies. These actuator assemblies utilize individual motors to drive power or ball screw devices and piston assemblies at each corner which generate the hydraulic pressure required to apply the brakes at that corner.
Although dry interface brake systems show enhancements over standard hydraulic brake systems by, for example, improving vehicle assembly and repair, dry interface brake systems may not be the braking system of choice at this time for various other reasons. For instance, to date, the dry interface brake systems do not utilize hydraulic braking as a secondary, or back-up, brake system to be used in the event of a failure of the electronics associated with the electronic control unit. As such, the dry interface brake systems of the prior art require advanced electronics to establish or provide a secondary brake system. Meeting customer expectations with respect to reliability may increase the costs associated with such advanced electronics for the secondary brake system.
Due to the challenges identified in the dry interface brake systems of the prior art, it is desirable to implement a xe2x80x9cwet interfacexe2x80x9d brake system including a brake system and method of controlling the brake system that provides secondary hydraulic braking during a de-energized operational mode of the brake system.
A brake system and method for controlling the brake system of the subject invention are disclosed. The brake system and method provide secondary hydraulic braking in a de-energized operational mode, such as a power failure, of the brake system. More specifically, the brake system includes a wheel brake assembly including a brake application device, such as a brake caliper or brake drum, used for decelerating a vehicle. The brake system further includes a master cylinder in fluid communication with the wheel brake assembly. The master cylinder operates the brake application device when the brake system is in the de-energized operational mode.
A first valve, known in the art as an isolation valve, is disposed between the master cylinder and the wheel brake assembly. Preferably, the first valve is a solenoid valve. In a normal, energized operational mode of the brake system, the first valve decouples the master cylinder from the wheel brake assembly such that hydraulic fluid from the master cylinder is prevented from operating the brake application device while the brake system is in the energized operational mode. On the other hand, when the brake system is in the de-energized operational mode, the first valve couples the master cylinder with the wheel brake assembly. As such, the master cylinder can operate the brake application device in the de-energized operational mode thereby providing the secondary hydraulic braking, when needed, through the master cylinder.
The brake system of the subject invention also includes an actuator assembly. The actuator assembly provides primary braking in the brake system of the subject invention. The actuator assembly, which is in fluid communication with the wheel brake assembly, includes a motor, a screw device, and a piston for operation of the brake application device. More specifically, a controller communicates with the motor to drive the screw device and the piston thereby generating the hydraulic pressure necessary for operation of the brake application device. While the brake system is in the energized operational mode, the operation of the brake application device by the actuator assembly is independent of the master cylinder as the master cylinder is isolated from the wheel brake assembly.
The brake system further includes a second valve which is also preferably a solenoid valve. The second valve is disposed between the actuator assembly and the wheel brake assembly. Preferably, the second valve is disposed in the actuator assembly between the piston and the wheel brake assembly. The second valve is a normally-closed solenoid valve such that the second valve is closed in the de-energized operational mode and open in the energized operational mode. When open, the second valve directly couples the actuator assembly with the wheel brake assembly to allow the actuator assembly to operate the brake application device. On the other hand, when the second valve defaults into the closed position, the second valve isolates the actuator assembly from the wheel brake assembly. As such, as described above, the actuator assembly is isolated, and the secondary hydraulic braking is provided directly through the master cylinder.
The second valve may also be de-energized into the closed position, even as the first valve is energized into its closed position, in order to maintain constant operational pressure to the brake application device. In such a case, the actuator assembly can then be deactivated thereby eliminating the need to supply current, i.e., power, to the motor which reduces overall wear on the motor.
Accordingly, the subject invention provides a wet interface brake system including a brake system and method of controlling the brake system that improves upon conventional dry interface brake systems by providing secondary hydraulic braking during a de-energized operational mode, such as a power failure, of the brake system.